> For the complete documentation index, see [llms.txt](https://hacking-notes.jord4n.pro/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hacking-notes.jord4n.pro/es/web/prototype-pollution/dom-xss-via-client-side-prototype-pollution.md).
# XSS DOM mediante contaminación de prototipos del lado del cliente
Notas sobre XSS DOM mediante contaminación de prototipos del lado del cliente para los laboratorios de PortSwigger Web Security Academy, incluido el flujo de explotación, pruebas de payload, validación en Burp Suite y contexto de mitigación.
#### XSS en el DOM mediante contaminación del prototipo del lado del cliente
#### Contexto del laboratorio
La aplicación es vulnerable a una **XSS en el DOM** activada por una **contaminación del prototipo** (del lado del cliente). / Propósito: **contaminar `Object.prototype`** con una propiedad útil, encuentra un **gadget** en el JS que la opera, y luego ejecuta `alert()`.
### 1) Observación de la contaminación del prototipo
#### Fuente de contaminación (en la URL)
Las propiedades pueden inyectarse en `__proto__` a través de los parámetros:
/?__proto__[foo]=bar
#### Control en la consola
Después de la navegación, se comprueba que la propiedad "fail" esté en todos los objetos:
```javascript
console.log({}.foo)
```
### 2) Código vulnerable del cliente
El sitio contiene una lógica de
```javascript
async function logQuery(url, params) {
try {
await fetch(url, {method: "post", keepalive: true, body: JSON.stringify(params)});
} catch(e) {
console.error("No se pudo guardar la consulta");
}
}
async function searchLogger() {
let config = {params: deparam(new URL(location).searchParams.toString())};
if(config.transport_url) {
let script = document.createElement('script');
script.src = config.transport_url;
document.body.appendChild(script);
}
if(config.params && config.params.search) {
await logQuery('/logger', config.params);
}
}
window.addEventListener("load", searchLogger);
```
Punto clave: el código lee **`config.transport_url`** y lo usa como **fuente de un `
```
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